1) Field of the Invention
The present invention relates to an in-plane switching liquid crystal display, more particularly to a liquid crystal display device suppress the shift of display colors due to aging and a liquid crystal display apparatus including the liquid crystal display device.
2) Description of the Related Art
Conventionally, an in-plane switching (hereinafter, “IPS”) image display apparatus is proposed, and practical application of this apparatus is progressed. The IPS image display apparatus has a configuration of controlling the orientation of liquid crystal molecules and displaying a predetermined image by applying an electric field to the liquid crystal molecules in a direction parallel to the substrate where a liquid crystal layer is disposed. As compared with a conventional image display apparatus that an electric field is applied in a direction perpendicular to the substrate, the IPS image display apparatus has superior voltage holding characteristic and a broader view angle due to the electric field direction.
FIG. 12 is a schematic view of a part of the conventional IPS image display apparatus. As shown in FIG. 12, the conventional IPS image display apparatus includes a transparent array substrate 101, a transparent counter substrate 102, and a liquid crystal layer 103 that is disposed between these substrates.
On the array substrate 101, there are disposed a common electrode 104, a pixel electrode 105, a data line 106, and a scan line (not shown). The liquid crystal molecules having the liquid crystal layer 103 has orientation. By controlling the electric field applied to between the pixel electrode 105 and the common electrode 104, an image can be displayed by utilizing the electro-optic effect of the liquid crystal.
Further, a color filter 107 is disposed on the counter substrate 102 to make it possible to display a color image. The color filter 107 has a function of transmitting only lights of wavelength corresponding to R (red), G (green), and B (blue) out of an incandescent light that is transmitted through the liquid crystal layer 103. FIG. 12 displays a green color layer 110 that makes the light of the wavelength corresponding to green pass through as an example.
A back light unit not shown that functions as a light source is provided beneath the array substrate 101, and irradiates a plane beam of an incandescent light to the array substrate 101. The liquid crystal layer 103 has a function of generating lights and shades on the screen by controlling a light transmittance of the irradiated incandescent light corresponding to the potential of the pixel electrode 105, thereby to display the image. Specifically, a predetermined potential is given to the pixel electrode 105 to generate an electric field between the pixel electrode 105 and the common electrode 104. The orientation of the liquid crystal molecules within the liquid crystal layer 103 is controlled in the electric field. The light transmittance is controlled following the change in the orientation.
The function of a light shielding layer 111 provided within the color filter 107 is explained. The electric field applied to the liquid crystal molecules within the liquid crystal layer 103 is generated not only between the pixel electrode 105 and the common electrode 104 but also between the common electrode 104 and the data line 106. The data line 106 has a predetermined potential irrespective of the potential of the pixel electrode 105. Therefore, the electric field attributable to the data line 106 is generated irrespective to the potential variation of the pixel electrode 105. Consequently, in the case of the IPS image display apparatus in the normally black mode, for example, the orientation of the liquid crystal molecule near the data line 106 changes due to the electric field attributable to the data line 106, even when a potential is not given to the pixel electrode 105 display black. As a result, the light that passes through the data line 106 and the common electrode 104 is discharged to the outside, and a black color having a color shading is displayed, which degrades the displayed image.
In order to shield the light that passes through the data line 106 and the common electrode 104, the light shielding layer 111 is disposed, thereby to suppress the degrading of the image quality. Particularly, the light shielding layer 111 is disposed in the area corresponding to the area where the common electrode 104 and the data line 106 are disposed within the color filter 107. With this arrangement, the leakage of the light that passes through between the data line 106 and the common electrode 104 to the outside is suppressed, thereby maintaining a high image quality (refer to Japanese Patent Application Laid-open Publication No. 9-101538, for example).
However, based on the provision of the light shielding layer 111, the conventional IPS image display apparatus has the following problems. First, the aperture ratio falls due to the provision of the light shielding layer 111.
While the light shielding layer 111 is provided to prevent the light not contributing to the image display from being leaked out, the light shielding layer 111 also shields a part of the light that passes through between the pixel electrode 105 and the common electrode 104 that is necessary for the image display. The incandescent light supplied from the back light unit contains a part of the component that proceeds to an inclined direction. Therefore, in order to shield this light as well, the light shielding layer 111 is disposed over a wide area. When a positioning error at the manufacturing time is considered, the area of the light shielding layer 111 further expands. Consequently, a part of the light that is necessary for the image display is also shielded near the end of the light shielding layer 111, which lowers the aperture ratio, resulting in a reduction in the brightness of the display image.
Degradation of the image quality due to the disposition of the light shielding layer 111 is also pointed out. Specifically, it is known that due to the accumulation of a charge near the boundary between the light shielding layer 111 and the green color layer 110, a third electric field is applied to the liquid crystal layer 103, which degrades the image quality. This phenomenon is considered attributable to a reduction in the specific resistance of the green color layer 110 due to the aging and a subsequent reduction in the light transmittance due to the accumulation of the charge, in the configuration shown in FIG. 12. Actually, as a result of an acceleration test of the color filter 107 carried out in a high temperature and high humidity condition, the inventors of the present application confirm that the specific resistance of 1015 Ω·cm at the beginning decreases to 1010 Ω·cm.
Further, as a result of an acceleration test carried out for the IPS image display apparatus built in with the color filter, it is confirmed that the light transmittance in the green color layer 110 is lowered extremely more than in other color filter due to the charge accumulation, and that the color tone of the total image shifts. FIG. 13 is a graph illustrating a change in a y value that indicates the intensity of the green color component of an image displayed in the IPS image display apparatus according to the conventional technique as a result of the acceleration test. In the present acceleration test, the intensity is measured for two different IPS image display apparatuses, and lines 13 and 14 are obtained. As is clear from the lines 13 and 14, the y value is lowered by about 0.02 as a result of the acceleration carried out for 100 hours. In general, when the y value changes by about 0.01, a color shift of the display image can be visually confirmed. A user recognizes the color shift in the IPS image display apparatus having the conventional configuration, which is not preferable.